Frits Paerels Is Among First to Glimpse Exotic Interior of a Neutron Star Through Series of Explosions

Frits Paerels

Through the fury of 28 thermonuclear explosions on a neutron star in the constellation Volans, or Flying Fish, about 30,000 light years away, a team of three scientists -- including Columbia astrophysicist Frits Paerels -- have gathered data and obtained a key measurement that can reveal answers for testing the current theories describing the fundamental nature of matter and energy. The breakthrough is featured in the November 7 issue of Nature.

"We have now bored our first small hole into a neutron star," said Paerels. "Unlike the Sun, with an interior well-understood, neutron stars have been like a black box."

The core remains of a star, once bigger than the Sun yet now small enough to fit inside Manhattan, a neutron star contains densely packed matter under forces that perhaps existed at the moment of the Big Bang but which cannot be duplicated on Earth.

Using data gathered by the European Space Agency's (ESA) XMM-Newton X-ray satellite, the scientists probed the star's interior by measuring for the first time how light passing through its centimeter-high atmosphere is warped by extreme gravity, called the gravitational redshift. The extent of the gravitational redshift, as predicted by Albert Einstein, depends directly on the neutron star's mass and radius. The mass-to-radius ratio, in turn, determines the density and nature of the star's internal matter, called the equation of state.

"We finally have data with enough light in it -- light that we are sure is coming from the surface of the neutron star -- that we can measure the fundamental properties of neutron stars, just as we do with ordinary stars," said Paerels.

The data was obtained from XMM-Newton X-ray satellite's X-ray detectors, which were designed by Columbia astrophysicists in collaboration with research institutes in the Netherlands, Britain, Switzerland and the United States.

Neutron stars form when stars several times more massive than the Sun run out of fuel and collapse. Scientists estimate that neutron stars contain the mass of about 1.4 suns compacted into a sphere only about 10 miles in diameter. At such density, all the space is squeezed out of the atoms inside the neutron star, and protons and electrons squeeze into neutrons, leaving a neutron superfluid.

By understanding the precise ratio of mass to radius, and thus pressure to density, scientists can determine the nature of this superfluid and speculate on the presence of exotic matter and forces within -- the type of phenomena that particle physicists search for in earthbound particle accelerators.

Today's announcement states that the neutron star in this binary star system, known as EXO 0748-676, has a mass-to-radius ratio of 0.152 solar masses per kilometer, based on a gravitational redshift measurement of 0.35. This provides the first observational evidence that neutron stars are indeed made of tightly packed neutrons, as predicted by theory estimating mass-radius, density-pressure ratios.

Co-author Mariano Mendez of SRON, the National Institute for Space Research in the Netherlands, said: "We have now established a means to probe the bizarre interior of a 10-mile-wide chunk of neutrons thousands of light years away -- based on gravitational redshift. With the fantastic light-collecting potential of XMM-Newton, we can now measure the mass-to-radius ratios of other neutron stars, perhaps uncovering a quark star."

A quark star is denser than a neutron star, thus with a different mass-to-radius ratio, in which neutrons are squeezed so tightly that they liberate the subatomic quark particles and gluons that are the building blocks of atomic matter.

To obtain its measurement, the team needed the fantastic radiance provided by thermonuclear bursts, which illuminate matter very close to the neutron star surface where gravity is strongest.

Gas from EXO 0748-676's companion star flows toward the neutron star, attracted by its strong gravity. The flow of gas forms a swirling disk around the neutron star, called an accretion disk. Thermonuclear bursts arise soon after gas slams onto the neutron star surface. This gas, pinned to the neutron star by gravity, spreads across the surface. As more and more gas rains down, pressure builds and temperature climbs until there is enough energy for nuclear fusion. This ignites a chain reaction that engulfs the entire neutron star within a second.

"It is only during these bursts that the region is suddenly flooded with light and we were able to detect within that light the imprint, or signature, of material under extreme gravitational forces," said NASA Goddard Space Flight Center's Jean Cottam, also a co-author of the study.

Periods of bursting, however, are unpredictable events. The team spotted the 28 bursts during a series of XMM-Newton observations of the neutron star totaling 93 hours. Because the force of gravity diminishes quickly with distance, scientists can only observe the effects of gravitational redshifting very close to the neutron star surface. And because X-rays are the predominant form of radiation near the surface, scientists require X-ray telescopes with large collecting area to make such measurements. XMM-Newton has one of the largest X-ray-light-collecting instruments now in space.